There is a long standing historical relationship between lighthouses and religious establishments as many of the earliest lights for the benefit of mariners were established as a result of charitable efforts by one church or another.  However, in the modern era, it is unusual to see an active light situated on top of a church.  The light in here is situated in a small cupola on top of the dome of the Lutheran Cathedral of Suomenlinna.  The church itself stands within the great sea defence fortress of Suomenlinna, sometimes referred to as "The Gibraltar of the North".  The church was built in 1854 as the church for the Russian Garrison of the fortress and, as such, it was a Russian Orthodox church topped off with the traditional onion dome which is such a distinctive feature of orthodox churches.  However, following Finland's independence from Russia in the 1920, the church became Lutheran and the onion dome was replaced by a more conventional dome on top of which the light is now situated.  The light displays four short flashes every fifteen seconds and, it may just be coincidence, but four dots represent the letter "H" in morse code and "H" represents Helsinki.  One source gives the impression that this was an intentional feature of the design but I haven't seen that confirmed anywhere else.